A gas laser oscillation apparatus of orthogonal excitation type contains an oscillator's housing with a sealed structure in which a laser medium gas such as a CO2 gas is enclosed (see Patent Document 1, for example). Inside of the oscillator's housing, there are provided electric discharge electrodes in use for the discharge excitation of the laser medium gas, a heat exchanger for cooling the laser medium gas, a blower for circulating the laser medium gas and the like. The gas laser oscillation apparatus of orthogonal excitation type functions as a laser amplifier, when windows are attached as substitutes for a total reflection mirror and a partial reflection mirror (see Patent Document 2, for example).
In recent years, Extreme Ultra Violet (EUV) light source devices of Laser Produced Plasma (LPP) type have been developed as light sources corresponding to a next generation exposure machine (see Patent Document 3, for example). CO2 gas laser oscillation apparatuses used for the driver light sources are in need of high power output and downsizing. As for laser oscillators used for laser processing and the like, the increase of the power output has become an issue for the improvement of processing ability (see Patent Documents 4-11, for example).